Surgical tools and instruments must possess certain characteristics in order to qualify for medical use. These characteristics include hardness, nick-free cutting edges and corrosion free surfaces. Such tools are conventionally manufactured using 400 series stainless steels. The 400 series stainless steels, or martensitic stainless steels, are low carbon stainless steels that are easy to sterilize and have high mechanical strength. However, such steels are relatively susceptible to corrosion and extreme care must be taken with their storage. The cost of medical tools and instruments made of these materials is extremely high.
The proper grain structure in the cutting edges of medical tools such as scalpels, scissors, elevators, curettes, vice grips, etc. is critical. The appropriate grain structure allows the instrument to properly cut bone surfaces during surgery and create an area where bone regrowth can find a good "grip" for the new bone fusion. Any nicks appearing on the cutting edge may be transferred to the bone being cut during surgery and may lead to trauma that will slow down post-surgery healing.
Unfortunately, the various requisite and desirable characteristics for medical tools and instruments are not currently available in a single type of tool or instrument. Although the strength and grain structure requirements are met by the current devices made of 400 series stainless steel, the costly tools and instruments have a very short lifetime due to their tendency to rapid corrosion.
The corrosion resistance of any material used for manufacturing medical tools and instruments is always quickly put to the test. For both historical and sanitary reasons, all such instruments and tools are "sterilized" under severe conditions following each use. Typically, medical tools and instruments are soaked in a concentrated solution of caustic, rinsed and then steam/gas sterilized in an autoclave between each usage. Both the soaking and autoclaving processes dramatically promote the oxidation or corrosion of metal surfaces. The costly medical tools and instruments that are currently used in surgery and medical applications are often made useless due to corrosion after only 5 to 10 uses.
This invention describes a method for applying a thin hard coating on surgical tools and instruments. The resulting tools have superior edge retention, superior corrosion resistance and long tool life.
Japanese Patent No. 59-76872 of Okubo describes a method for manufacturing metal medical and dental instruments such as razor blades, scalpels, scissors and forceps. A coating of nickel or nickel alloy, chromium or chromium alloy, or a coating of gold alloy is applied to the surface by wet plating methods. Next, a hard coating such as a nickel phosphorous alloy is applied to the surface of the instruments. The last step of the invention is the application of a hard film of titanium nitride by an ion plating method.
One of the objectives of the inventors of the Japanese patent was to produce less costly medical tools and instruments. In so doing, their invention contemplates the use of less expensive, relatively low quality substrates of iron or iron alloys. The complicated procedure of applying several coating layers is necessary due to this choice of substrate material. The direct application of titanium nitride on such substrates would result in poor adhesion and corrosion resistance due to galvanic cell formation between the iron of the substrate and the nitride film, making any such tools highly undesirable for medical purposes. In addition, any process calling for several steps utilizing wet plating methods is environmentally suspect.
Swiss Patent No. 5297/78 of Hintermann et al. describes cutting instruments specifically designed for use by Ophthalmologists. The instruments are made of steel that has been coated with a titanium carbide, nitride or carbonitride film. In the completed article, the cutting edge remains uncoated. The patent suggests that deposition of the coating may be accomplished by vacuum evaporation, ion plating or chemical deposition processes.
The chemical deposition process is accomplished by a gas phase chemical reaction generally occurring at extremely high temperatures such as approximately 1000.degree. C. At such elevated temperatures, there would be a significant reduction in the mechanical strength and wear properties of the steel substrate. Chemical deposition processes are, therefore, unsuitable for coating tools and instruments for medical or surgical purposes.
The titanium nitride or titanium carbide films obtained by vacuum evaporation or conventional ion plating techniques, as suggested in the Swiss patent, generally have columnar structure. Coatings having such columnar structure generally must be at least 3 microns thick, will contain voids, and will be a relatively low density surface. This results in relatively poor corrosion protection and wear resistance. Film thicknesses over 3 microns also contribute to poor film adherence due to the high density of compressive stresses in these coatings.
In order for coated instruments to be suitable for surgery, the coatings should have excellent corrosion resistance, wear resistance and good adhesion to bare substrates. This can be achieved by a suitable selection of coatings and coating processes. One superior coating process is the cathodic arc plasma deposition ("CAPD") method.
CAPD has unique features relative to other ion plating techniques. Among these are the ability to generate a high degree of target molecule ionization and higher ion energies. These features lead to the creation of high density coatings with superior bonding to substrate characteristics. High density coatings have relatively fewer voids, and can be applied in much thinner layers while still assuring complete surface coverage.
A suitable film with higher hardness and wear properties than titanium nitride/carbide would also be very desirable. One such coating is zirconium nitride, which has higher hardness than titanium nitride and also possesses a lower co-efficient of friction. Both of these characteristics would suggest that zirconium nitride would be a better candidate material for medical tools and instruments than other metal compounds currently suggested in the literature.